Fibromyalgia syndrome (FMS) is the most frequent cause of chronic, widespread pain, estimated to affect 2–4% of the population. FMS is characterized by a generalized heightened perception of sensory stimuli. Patients with FMS display abnormalities in pain perception in the form of both allodynia (pain with innocuous stimulation) and hyperalgesia (increased sensitivity to painful stimuli). The syndrome, as defined by the American College of Rheumatology's criteria, involves the presence of pain for over 3 months duration in all four quadrants of the body, as well as along the spine. In addition, pain is elicited at 11 out of 18 “tender points” upon palpation. Other associated symptoms include fatigue, nonrestorative sleep, and memory difficulties.
Chronic fatigue syndrome (CFS) is a debilitating disorder characterized by profound tiredness or fatigue. Patients with CFS may become exhausted with only light physical exertion, and must often function at a level of activity substantially lower than their capacity before the onset of illness. In addition to the key defining characteristic of fatigue, CFS patients generally report various nonspecific symptoms, including weakness, muscle aches and pains, excessive sleep, malaise, fever, sore throat, tender lymph nodes, impaired memory and/or mental concentration, insomnia, and depression. Like patients with FMS, patients with CFS suffer from disordered sleep, localized tenderness, and complaints of diffuse pain and fatigue.
There are two widely used criteria for diagnosing CFS. The criteria established by the U.S. Centers for Disease Control and Prevention include medically unexplained fatigue of at least six months duration that is of new onset, not a result of ongoing exertion and not substantially alleviated by rest, and a substantial reduction in previous levels of activity. In addition, the diagnosis involves the determination of the presence of four or more of the following symptoms—subjective memory impairment, tender lymph nodes, muscle pain, joint pain, headache, unrefreshing sleep, and postexertional malaise (>24 hours). Reid et al., 2000, British Medical Journal 320: 292–296. The diagnostic criteria from Oxford includes severe, disabling fatigue of at least six months duration that affects both physical and mental functioning and the fatigue being present for more than 50% of the time. In addition, the diagnosis involves the determination of the presence of other symptoms, particularly myalgia and sleep and mood disturbance. Reid et al., 2000, British Medical Journal 320: 292–296.
Owing to their common symptomology, FMS and CFS are thought to be related. However, they manifest different major symptoms. Whereas pain is the major symptom reported by patients with FMS, fatigue is the major symptom reported by patients with CFS. Given their relatedness, these two indications have been treated with the same medications. Some of the common medications currently employed to treat CFS and/or FMS include, but are not limited to, analgesics, hypnotics, immune suppressants, various other prescribed medications, and an array of non-prescription medications. Although a broad array of medications are used in FMS and CFS patients, no single pharmacological agent or combination of agents is effective in the treatment of either of these disorders. Thus, due to the lack of effective treatment regimens for FMS and/or CFS, there is a need to develop effective treatments.
Pain is associated with a variety of different underlying illnesses or injuries. Pain may be either acute or chronic. Chronic or intractable pain is often endured over many years or decades. Patients suffering from chronic pain often develop emotional problems which can lead to depression and in the worst case, attempted suicide. Long lasting pain often occurs particularly in joints, in muscles, connective tissue and in the back. In the United States alone, chronic pain causes a loss of more than 250 million working days per year. A patient is considered to have chronic pain when complaints thereof last longer than six months. In the course of time, chronic pain may form an independent clinical syndrome.
Most analgesic agents in use today are not always effective, may produce serious side effects and can be addictive. Hence, there is a demand for more active analgesic agents with diminished side effects and toxicity, and which are non-addictive. The ideal analgesic would reduce the awareness of pain, produce analgesia over a wide range of pain types, act satisfactorily whether given orally or parenterally, produce minimal or no side effects, and be free from the tendency to produce tolerance and drug dependence.